In electric power generation, such as wind power generation, one of the established configurations is to use a full power back-to-back converter system. In such systems, the generator, e.g. wind turbine, is controlled by a voltage source converter to deliver power to a DC link, and another voltage source converter connected to this DC link is controlled to feed the power to the grid.
In conventional full power back-to-back voltage source converter power generation systems, the generator converter is rated to have the same levels of voltage and current as the generator. Further, the converter is operated in pulse width modulation (PWM) mode in order to provide the required voltages and currents to the generator. With concern to the current harmonics and torque ripples in the generator and generator control dynamics, the PWM switching must occur at a relatively high rate, such as 2.5 kHz. As each switching causes additional power losses in the power components (and thus heat), a higher switching frequency will not only cause reduced efficiency and increased requirements for converter cooling, but also more importantly result in a shorter lifetime for the converter. The latter is particularly undesirable for offshore wind power applications.
Given the existing power component techniques, it is difficult to handle the required power level for wind power generation with a single power unit. Instead, modular converters are often used, where several power units are arranged in parallel for providing the required power/current level. In order to improve the current sharing between paralleled modules, especially during switching transients, special techniques have been applied, such as pulse insertion or dropping. Nevertheless, some compromises are still placed to allow less ideal switching, and more modules have to be used to create the needed current/power margins. However, an increased number of components means higher cost and less reliability on the system level.
The technical challenges to the conventional converter system are becoming severer as wind power generation is heading towards even higher voltages and larger power levels.
Several alternative lower cost converter topologies utilizing diode rectifiers and boost converters in the DC link have been discussed in the literature. However, the generator stator currents cannot be controlled directly and independently using a diode rectifier. Furthermore, known torque ripple methods using a boost converter in the DC link suffer from limited controllability and increased latency due to the diode rectifier.
Accordingly, there may be a need for an electric power generator without the above drawbacks, in particular an electric power generator that is capable of reliably meeting the demands regarding high voltage and power levels and which at the same time is relatively cheap and easy to manufacture and implement.